I want to learn Complete Top-down Assembly modeling and all the tricks/technique to get proper Bill of Material.

I want to design a big project but always I go with bottom to top, Now realize that sometimes it very hard to get any reflection in BOM by any change in design Moreover, by putting a lot of mates makes slow down my assembly.

1: Could anyone share any good website or any video series through I can learn how to design Top-down assembly.

2: Surface flatten command only activate in the premium version?

3: which technique is efficient for modeling any assembly?

Much appreciation in advance.